A Global Optimization approach of membrane gas separation processes, based on a general process superstructure including a wide array of possible configurations, and solved by a Nonlinear Programming formulation is presented. The capacity of the proposed approach to provide optimal configurations at minimum separation cost is first validated by comparing the obtained solutions with those of a reference study in the domain. The optimization approach is then applied to the optimization of CO2 capture from blast furnace gas considering multistage processes with up to four membrane stages. The optimal process configuration and main process variables, upstream and downstream pressure and membrane area, are determined for processes with CO2 recoveries of 90%, 95% and 99% and N-2 residual contents of 1%, 0.5% and 0.1%. The resulting separation cost is in the range of 29-45 FUR/ton CO2 based on a NETL type cost model. Two stage permeate cascades (enrichers) with retentate recycle are shown to be the optimal configuration for N-2 residual contents down to 1% at any recovery and down to 0.5% at 90% recovery. For larger recovery or purity levels, three stage processes offered the lowest separation cost. Four stage processes offered no marked improvement over three stage processes.