The gas which can’t be liquefied will be designed to go back to combustion system. All the gases are recycling to either drying Heat, pyrolysis heat or used as a fluidizing process. However, the vent is given for startup of the process.
As Prem said, the gas is generally not discarded as the energy content can be significant and therefore combusted and used to heat the process indirectly. No advanced system would just dump the gas. Initially during startup before steady state is achieved, then the low quality gases might be discarded, but they should be flared or passed through a thermal oxidiser.