A one-dimensional nonequilibrium model for multicomponent condensation is used to simulate a vertical single-pass shell-and-tube heat exchanger in an industrial gas-phase polyethylene reactor system. Starting the calculation at the top of the exchanger, the model can predict temperatures at the bottom of the exchanger within an accuracy of +/- 5 K as compared to three sets of industrial data. Sensitivities of model predictions were analyzed, including uncertainties associated with physical and transport property estimates, step size, and convergence criterion. Model predictions are not particularly sensitive to the estimation errors of physical and transport properties if K values are calculated using an equation of state applicable to both liquid and vapor phases. Effects of operating conditions on heat removal from polyethylene reactors were investigated for an existing process. It was quantitatively demonstrated why and how severely noncondensable gases impede condensation hear transfer. The level of noncondensable gases and the cooling wafer temperature are the two most important factors influencing the heat-removal rate. Replacing a portion of noncondensable gas, such as N-2, with a condensable fluid that is inert to polymerization reactions can substantially increase the heat-removal rate from the reactor, thereby allowing for an increase in polymer production rate.