A generalized steady-state population balance model is developed to investigate the particle-size distribution developments in gas-phase fluidized-bed olefin polymerization reactors. The model rigorously accounts for the combined effects of particle growth, attrition, elutriation and agglomeration for both uniform-size and size distributed catalyst feed. Model predictions are found to be in agreement with recent experimental results on particle elutriation and agglomeration in polyethylene fluidized beds. Simulation results show that the rate of particle agglomeration exhibits an exponential increase with the bed temperature. Furthermore, particle elutriation is influenced by the agglomeration process which largely reduces the number of small-size particles. It is also shown that due to the large polymerization rates exhibited in gas-phase olefin polymerization processes and to the near-well-mixed bed behaviour, the feed catalyst size distribution strongly affects the particle-size distribution in the bed. The population balance model developed in this paper proves to be a unique tool for simulating the effects of different process parameters on the polymer size distribution in gasphase fluidized-bed olefin polymerization reactors.