An approximate modeling technique for spiral-wound permeators separating binary gas mixtures is developed. The approximate model is derived directly from a standard fundamental model [1] by assuming that the residue now rate is constant in the direction of permeate flow. This assumption reduces the original boundary value problem to a more computationally tractable problem involving a small number of nonlinear algebraic equations. Theoretical justification for the modeling technique is obtained via comparison to one-point collocation. A nonlinear programming method for estimating unknown model parameters from experimental data is proposed. The approximate modeling and parameter estimation techniques are evaluated for the separation of CO2/CH4 mixtures.