In the present study, a steady-state population balance model is developed for the prediction of the particle size distribution in ethylene copolymerization FBRs operating under moderate particle agglomeration conditions. To calculate the growth rate of a single particle under internal and external heat and mass transfer limitations, the polymeric flow model (PFM) is employed. The PFM is solved together with a steady-state particle population model to predict the PSD in the FBR. An empirical correlation is developed for the calculation of the agglomeration kernel in terms of the particle surface temperature and polymer softening point based on reported experimental measurements. Under complete mixing conditions of solids in the bed and a uniform catalyst feed, it is shown that internal and external mass and heat transfer limitations can have a strong impact on the calculated PSD in the bed. Furthermore, the effect of the bulk polymerization temperature and the comonomer to monomer molar ratio on the extent of particle agglomeration in ethylene copolymerization FBRs is investigated.