A three-dimensional (3-D) computational fluid dynamics model, coupled with population balance (CFD-PBM), was developed to describe the gas-solid two-phase flow in fluidized-bed polymerization reactors. The model considered the Eulerian-Eulerian two-fluid model, the kinetic theory of granular flow, the population balance, and heat exchange equations. First, the model was validated by comparing simulation results with the classical calculated data. The entire temperature fields in the reactor were also obtained numerically. Furthermore, two case studies, involving constant solid particle size and constant polymerization heat or evolving particle-size distribution, polymerization kinetics, and polymerization heat, were designed to identify the model. The results showed that the calculated results in the second case were in good agreement with the reality. Finally, the model of the second case was used to investigate the influences of operational conditions on the temperature field.