A polymerization reactor connected to a semi-flooded horizontal condenser is presented. A model of the process was developed, taking into account the influence of non-condensable gases in the system. The results obtained show that the model developed was able to reproduce the major dynamic characteristics, even with the presence of 'non-condensable gases. It is shown that the non-condensable gases have great influence on the reactor state, which never reaches a steady-state; these gases accumulate in the system, increasing the pressure and temperature, and reducing the area of contact and the mass of liquid, therefore needing to be purged regularly, otherwise the system will collapse.