A steady-state model incorporating interactions between separate bubble and emulsion phases in a fluidized-bed polyethylene reactor was developed by Choi and Ray [Chem. Engng Sci. 40, 2261-2279 (1985a)]. Correlations for maximum stable bubble size indicate that bubbles within the bed are considerably smaller than those in their original model. In the paper, the influence of bubble size and superficial velocity on reactor operation are examined. It is shown that bubble size critically influences the rate of heat and mass transfer within the bed, and when the bubbles are as small as those predicted by the maximum stable bubble size correlations, there is little or no resistance to the transfer of monomer and heat between the phases. A simplified well-mixed model is developed to describe reactor operation in the limiting case where there is no difference between bubble and emulsion gas temperatures and concentrations. The differences between the predictions of temperature and monomer concentrations of the two-phase and simplified models are less than 2 or 3 K and 2 mol%, respectively, in the operating range of industrial interest. Therefore, a simple back-mixed model is appropriate for predicting temperature and concentration in the gas phase of industrial fluidized-bed polyethylene reactors.