Retrograde condensation happens in mixtures of components with substantially differing pure component vapor pressures. As pressure decreases in the retrograde region, a liquid phase forms, which is counter-intuitive based on pure component behavior. Further decreases in pressure cause the mixture to become one vapor phase again. The simplest conceptual understanding is to think of the mixture starting in a supercritical-like phase. In this state, the light molecules interact with the heavy molecules and prevent them from condensing as a liquid as they would do if there were no light molecules present. The molecular densities are high enough that the light molecules essentially solvate the heavy ones in a supercritical-like (relatively dense) phase. As pressure decreases, molecular density decreases and the tendency of the heavy phase to condense because of molecular attractions overwhelms the ability of the light phase to solvate the heavy phase. Themolecules form a liquid phase that is rich in the heavy component(s). Further decreases in pressure cause the condensed heavy phase to vaporize as one would expect for pure components. 

Retrograde condensation happens in mixtures of components with substantially differing pure component vapor pressures. As pressure decreases in the retrograde region, a liquid phase forms, which is counter-intuitive based on pure component behavior. Further decreases in pressure cause the mixture to become one vapor phase again. The simplest conceptual understanding is to think of the mixture starting in a supercritical-like phase. In this state, the light molecules interact with the heavy molecules and prevent them from condensing as a liquid as they would do if there were no light molecules present. The molecular densities are high enough that the light molecules essentially solvate the heavy ones in a supercritical-like (relatively dense) phase. As pressure decreases, molecular density decreases and the tendency of the heavy phase to condense because of molecular attractions overwhelms the ability of the light phase to solvate the heavy phase. Themolecules form a liquid phase that is rich in the heavy component(s). Further decreases in pressure cause the condensed heavy phase to vaporize as one would expect for pure components. 

i am agreeing with professor Baxter, this region is between critical and cricondenbar points.

http://pubs.acs.org/doi/abs/10.1021/ie50366a018?journalCode=iechad

Here it is the communication ( Communications from the Laboratory of Physics at the University of Leiden ) of J.P. Kuenen where the expression "retrograde condensation" was introduced:

https://www.lorentz.leidenuniv.nl/history/KOL_archive/Communications/01_12/4a_Kuenen.pdf

when the pressure declines due to the production, the kinetic energy of gas molecules increases and hence the collisions of of heavy components (C7+) and this in turn leads to the condensation of the heavy components.

If the pressure decreases more and more the condensate will be vaporized again.

I am sorry for my delay... thank you all to answering my question, but I have another question from Larry L.Baxter: what will cause heavy molcules and light ones to interact in such an special case... do you mean that these assembles will produce any discharge pressure to the system??

The heavy and light components interact at a molecular level, meaning they behave differently than they would if you just add the pure component behaviors such as vapor pressure and dew point. Specifically, the heavy molecules cause some of the light molecules to condense with them because of their mutual attraction, changing the point at which both condense relative to what would happen if they were pure components.

There is nothing special in retrograde condensation from a physical point of view,   intermolecular forces are responsible for this. I have read that this was a source of controversy and that  J.D. Van der Waals wrote a letter to K. Onnes, supervisor of J.P. Kuenen at Leiden, that this new behaviour was an implicit  consequence of his theory applied to mixtures.

thanks... but I can not explain double retrograde region with this theory of supercritical phenomena... can you tell me what will happen there??

and also I am not looking for a physical interaction, by the word interacion actually I mean the chemical ones...thanks anyway

I recommend you to read this paper:

http://ijens.org/Vol_18_I_05/180505-3737-IJMME-IJENS.pdf

Best regards