In the present study recent developments in modeling gas-phase catalyzed olefin polymerization fluidized-bed reactors (FBR) are critically reviewed. A new FBR model is developed to account for the effect of varying bubble size with the bed height on the reactor dynamics and the molecular properties of the polymer product. A comprehensive kinetic model for ethylene copolymerization in the presence of a multisite Ziegler-Natta catalyst is considered to describe the molecular weight developments in the FBR. The bubble-growth model developed in the present study is subsequently compared with two well-known FBR models, namely, the well-mixed and the constant bubble size model. The effect of important reactor parameters such as superficial gas velocity, maximum stable bubble size, mean particle size, catalyst injection rate, monomer/comonomer feed ratio and temperature of the feed stream, on the dynamic and steady-state behavior of the FBR is investigated. It is shown that the maximum stable bubble size, a critical parameter in the constant bubble size model, turns out to be less important when the bubble size is allowed to vary with the bed height. For typical industrial operating conditions, the constant bubble size model consistently overpredicts the emulsion phase temperature and monomer conversion, while the well-mixed model underestimates them. On the other hand, the bubble-growth model shows an intermediate behavior since it represents a more realistic description of the gas phase in the bed.