We have proposed the methodology for analyzing fluidic phase behavior and transport processes in large-scale porous media, which takes into account the effects of interfaces in nanoscale confined spaces, based on the mean-field lattice gas model and Monte Carlo method. More specifically, the mean-field lattice gas model computes capillary condensation in nanopores, followed by gas diffusion simulations using the Monte Carlo method. The proposed methodology is fairly versatile, not limited to specific applications such as the randomly packed spheres or micro-porous and catalyst layers of a polymer electrolyte fuel cell which the present study has dealt with.