Analytical gas-permeation models for predicting the separation process across the membranes constitutes an important and necessary step in understanding the overall performance of the membrane modules. Therefore, solution of the nonlinear ordinary differential equations that govern the performance of the membrane modules for gas separations is necessary. Approximate and asymptotic methods are useful, simple and rapid methods in the design and comparison of gas separation processes, but these methods have certain application limits. In this work, a new combinational approach (combination of asymptotic and approximate methods: CAAM) was applied for predicting the performance of the nanometric tubular ceramic membranes in the separation of binary gas mixtures with cocurrent and countercurrent flow patterns. Also, the exact (numerical) solutions of the governing equations using the fourth-order Runge-Kutta technique were proposed. The comparison of the results showed a good agreement between the exact solution and the CAAM method over the whole range of selectivities (1 <= alpha <= infinity). The accuracy of this method was verified by comparison the predicted results with different literature experimental data and mathematical models. Also, more detailed parametric studies were done and a generalized method was proposed for determining the application limits of the simplified methods in our combinational approach. It can be concluded from this work that the CAAM method can provide excellent short-cut, preliminary design information. (c) 2006 Elsevier B.V. All rights reserved.