Fluidized bed catalytic cracking unit (FCCU) is one of the most important processes in refinery, sometimes called as the heart of refinery, since it is the main responsible for the profitable conversion of heavy gas oil into commercial valuable products. In this study we've seen the impact of rising temperatures in the reactor plenum (Reactor Plenum Temperature) on the production of polypropylene and percentage of conversions in the FCC unit. According to the simulation results, we can increase the production of propylene and increase the feed conversion during the reaction with rising temperatures in the reactor plenum (Reactor Plenum Temperature).