A reaction calorimeter was built to follow slurry-phase polymerizations of ethylene using different types of supported catalysts. It was shown that heat flow calorimetry, employing a high-gain observer for the evaluation of the initial conditions was an extremely useful tool for the measurement of on-line reaction rates, and a study of the influence of different parameters such as the stirring rate or solid content in real time. It was shown that if one uses solid contents under 30% (volume) then it is not necessary to account for the influence of this quantity on the overall heat transfer coefficient.