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The CO2 permeance and CO2/CH4 selectivity trade-offs of spun hollow fiber membranes (SHFMs) out of cellulose acetate (CA) as a model system were scrutinized via manipulating the system draw down ratio over precursor viscosity, DDR/ η 0 , and feed gas pressure. They were correlated through developing a phenomenological model, which under the high pressure of feed gas, and low DDR/ η 0 led to simultaneous high permeance/high selectivity output. Non Equilibrium Lattice Fluid (NELF) model was applied to estimate the permeance and ideal selectivity of SHFMs in different DDR/ η 0 regimes and low penetrant upstream pressure. Accordance of the model predictions based on the system DDR/ η 0 with the experimental data was improved by considering a magnifying coefficient, which implied the role of upstream feed gas pressure. Schweitzer's Self-consistent Nonlinear Langevin Equation (SCNLE) model was also used for the first time in the membrane separation field. Accordingly, the observed CO2 permeance of spun hollow fiber membranes versus their DDR/ η 0 was found proportional with the penetrant size over polymer chain Kuhn length ratio. It was also noticed that hollow fiber shrinkage dependence on DDR/ η 0 , was proportional with its corresponding selectivity change. [ABSTRACT FROM AUTHOR]