Fick's first law is generally accepted for describing the transfer of gas mixtures through pore-free dense polymeric membranes. Typically, the permeation flux is made up of two parts, namely, that resulting from the bulk motion and that resulting from diffusion. For simplicity most hollow fiber permeator models consider only the diffusion part; however, neglecting the frame of reference contribution can be erroneous in the case of mutlicomponent mixtures when the permeation flux of one of the permeants is much higher than the others. A comparative theoretical study is performed on the influence of two transport models on gas separation hollow-fiber modules: the diffusion and the frame of reference models. Simulations are performed for CO2/CH4 separation in 6FDA-TADPO polypyrrolone membranes. The comparison is made in terms of CH4 recovery and membrane selectivity under common design and operating conditions. It is shown that the differences in performance can be as high as 25% for membrane selectivity and 10% for CH4 recovery, especially for high CO2 concentrations and high pressures.