A mathematical model was developed to simulate an industrial-scale continuous process of polyamide-6,6 polymerization composed by two reactors in series: a coiled tubular reactor operating partially under onephase flow and partially under two-phase flow conditions, followed by a continuous stirred tank reactor. The mass, heat, and momentum balances for the single and two-phase flows were considered in the tubular reactor model. Water and hexamethylenediamine evaporation rates were evaluated according to the thermodynamic vapor-liquid equilibrium equations. Frictional, gravitational, and accelerational terms, as well as fittings for tube expansions, were considered in the pressure drop calculations. The model developed is an improved version of a previous one (Giudici et al., Chem. Eng. Sci. 1999, 54, 3243-3252) and was tested and validated with industrial data under a wider range of conditions, including the effect of catalyst.