At present, the gas composition in a polyethylene reactor is only available from a gas chromatograph with a 6-minute update. In this study, a mixed flow reactor model was developed to simulate the process in real time, using measurements of the reaction conditions and feed rates. Initial open-loop tests revealed that the prediction was in advance of the GC output by 450 seconds. A Smith predictor was then employed to establish the error between appropriately delayed composition predictions and the corresponding measurements, and to correct current predictions using this offset. Combination of this passive offset with recursive filtering of predicted compositions, similar to Kalman filtering, proved advantageous. Dual-rate sampling provided fast model responses to the process inputs within the GC cycle period.