This article describes a mathematical model for the finishing stage of nylon-6,6 polycondensation in a twin-screw extruder reactor. In the model, the extruder is conceptually divided into two regions. The first one is the partially filled degassing zone, which is operated under low pressure and where the evaporation of water from the polymer takes place. The rate of evaporation is considered to depend on an overall mass transfer coefficient and is limited by the water-polymer physical equilibrium. In the second region, which is fully filled, the polymer flow is assumed to be plug-flow and, in this region, the reversible polycondensation reaction occurs, as well as degradation reactions. A comparison with experimental data obtained in an industrial plant shows fairly good agreement with model predictions after optimal fitting of the rate coefficients.