A rigorous model of the cooling circuits in industrial scale suspension PVC reactors has been formulated and solved numerically by using the method of characteristics. Dedicated experimental tests have been performed with pure water in the reactors in order to extract reliable heat transfer data. The model has been tested for on-line prediction of the reaction rate and conversion. A comparison is made with a kinetic model, previously published by Kiparissides et al. (1997 Ind. Engng Chem. Res. 36, 1253-1267). Results show that the predicted reaction rates based on the cooling circuit model contains significant noise due to the large heat capacity of the suspension and limited resolution in the temperature measurements. Filtering of the reaction rates eliminates the problem, but introduces undesired time delay. The kinetic model shows good agreement with predicted reaction rates. Combined with conversion estimates based on the energy balance the kinetic model has been found to offer a reliable on-line estimate for the reaction rates.