An Eulerian–Eulerian two-fluid coupling the population balance model (PBM) is developed to describe the gas–solid two-phase flow in a multizone circulating polymerization reactor (MZCR). The polymerization kinetics is also incorporated into the coupled model by using a user-defined function (UDF). The model is first used to predict the entire field in the MZCR with considering the polydispersity of solid phase. Furthermore, the temperature field is obtained numerically with considering polymerization reaction, and the effect of inlet gas temperature on the temperature field is also investigated. Finally, the model is adopted to distinguish between the flow behaviors in a circulating fluidized bed reactor (CFBR) and a MZCR. The simulated results show that the flow behavior in the MZCR with polydisperse solid phase is different from that with uniform particle size and the inlet gas temperature has great effects on the temperature distribution. Moreover, the simulation results also show that the differences in between CFBR and MZCR are mainly embodied in the temperature distribution.