Glassy polymeric membranes are widely used in the separation of gas mixtures. Typically, the permeability of these membranes has a pressure and composition dependence that is well described by the dual-mode transport model. Nevertheless, for simplicity, most of the hollow-fiber permeator models only consider constant permeability, which can lead to inaccurate results. A comparative theoretical study is performed on the influence of two different membrane mass transport models on gas separation hollow-fiber modules: the constant permeability and the dual-mode transport models. The comparison is performed in terms of the recovery and purity of the faster gas, under different design and operation conditions. Simulations are performed for the He/CH4 separation in a polycarbonate membrane. It is shown that the differences in performance exhibited by the two models can go up to 10% for recovery and 5% for purity, particularly for high-permeate pressures. Pressure drop on the retentate and permeate sides is analyzed. Two pressure drop models based on Hagen-Poiseuille equation are considered and compared: constant viscosity, evaluated for feed conditions, and composition-dependent viscosity. It is concluded that neglecting viscosity composition dependency can lead to errors up to 20% in pressure drop calculations, consequently affecting the hollow-fiber performance in terms of recovery and purity.