The attainable-region approach for reaction, mixing, and separation is applied for step-growth melt polycondensations. A concentration-based formulation is applied to develop hybrid reactor−separator models for the two-phase continuous stirred tank reactor and plug-flow reactor with simultaneous vapor removal. The evaporation of the volatile byproducts, which is typically limited by liquid-phase mass transfer, is characterized with a Thiele modulus. A reaction−separation vector that satisfies the same geometric properties as the reaction vector is defined, so that a candidate attainable region can be constructed by following a known procedure for reaction−mixing systems. The technique is demonstrated on the industrially important polycondensation step in the production of Nylon 6,6. The effect of temperature, pressure, and Thiele modulus on the candidate attainable region for number-average molecular weight is analyzed.