Comprehensive modeling of the gas-phase polyethylene reactor was developed by combining discrete element method (for predicting motion of particulates), Navier-Stokes equation (for predicting velocity fields of gas phase), mass conservation equation (for predicting concentration of reactants), and energy conservation equation (for predicting temperature). A comprehensive kinetic mechanism was used to evaluate the rate of ethylene and 1-butene copolymerization reactions. Simultaneous solution of these equations provided information about concentration, temperature, and velocity fields and their effect on the final performance of the reactor, as well as detection of possible hot spot zones. The effect of critical reactor operating parameters (such as inlet gas velocity and operating pressure) on the temperature distributions of gas and particles in the bed was also studied to investigate the possibility of hot spot formation in the reactor. The results showed that bed temperature decreases with increasing inlet gas velocity. Also, the bed temperature profile decreases at higher pressures due to more efficient contact between gas and particles.