Thermal stability is a crucial issue for fluidized-bed ethylene-polymerization reactors, especially when the particle-aggregation probability increases as a result of the high reaction temperature or operation under condensed mode. To determine the mean particle size, a population-balance model (PBM) considering particle growth and particle aggregation was implemented in the present study. Then, an integrated model including the two-phase model (TPM), the PBM, and an evaporation model was proposed to describe polymerization of ethylene in liquid-containing fluidized-bed reactors. The results demonstrate that the particle-growth rate and catalyst particle size dominate the particle size of the final product. Thermal stability of fluidized-bed reactors is mainly affected by the catalyst activation energy, the superficial-gas velocity, and the condensed-liquid flow rate.