you have to export all data from Fluent to CFD POST. some of them exported normally and the others have to be selected from FILE menu in Fluent 

I'm not sure sure if I understood/interpreted your question correctly - but after solving if you open your model up in post, you can click on the function calculator and select all the walls, then select the Avg or area(Avg) function, then select the variable Wall Heat Flux.   This will give you the average heat flux of all your walls.  Alternatively you can create a monitor prior to solving with the same selections and this will allow you to monitor the total heat flux during the solve and output a graph.    To make it easier to select all the walls you are concerned with, within Design Modeler you can also select all the walls and create a 'Named Section' and use this in pre and post for faster selection.  

Mark, thank you for your answer. I think the way that you describe just give me the average heat flux. Because, the heat flux unit is watt/area and since I looking for watt, it must multiplied by each element area by heat flux in that element to get total heat transfer in the area. I think I asked the wrong question, actually I look for heat transfer from a surface.

you have to export all data from Fluent to CFD POST. some of them exported normally and the others have to be selected from FILE menu in Fluent