A mathematical model was developed to simulate the industrial process of nylon-6,6 polymerization. The process is carried out mainly in a tubular reactor under two-phase flow conditions ("flasher tube"). The model included equations for the single phase flow and the two-phase flow. The beginning of the two-phase region is determined as the point where the vapor pressure equals the total pressure. Pressure drop calculations account for frictional, gravitational, and accelerational terms, as well as for the head losses in fittings and tube expansions. Available corrections for helical coiled tubes and npn-Newtonian fluids are taken into account. After parameter adjustment, the model was validated by comparison with industrial plant data and then successfully applied to debottlenecking studies in the industrial plant.