A population balance model is developed to investigate the steady state particle size distribution developments in a gas-phase fluidized-bed ethylene polymerization reactor. The model considers the combined effects of particle growth and elutriation for size distributed prepolymer feed. In the proposed model, the bed is divided into several sections consisting of bubble and emulsion phases. The population balance differential equations derived for each section were simultaneously solved to determine the density function of the size distribution of the polymer particles in each section. The model is able to estimate the axial profile of particle size distribution along the reactor height. It was shown that the mean size of the particles is larger at the bottom of the bed and becomes smaller when moving toward top of the reactor. The size distribution of the polymer particles in the product becomes sharper and their mean size decreases by increasing the superficial gas velocity. Also, mean size of the particles in the product increases by increasing the temperature of the reactor. (C) 2012 Elsevier Ltd. All rights reserved.