Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) is coupled with Young-Laplace equation to investigate and represent fluid-phase equilibria in nanosize pores. The calcu-lated pure-substance properties at capillary condensation (critical temperature, critical pore ra-dius, surface tension, condensed-phase equilibrium pressure, and saturated densities) are found to be consistent with experimental data, theoretical models, and molecular simulation. The informa-tion obtained for pure substances can then be applied to predict the phase equilibria of fluid mix-tures in porous mediums. The promising results presented in this work establish a strong plat-form for further development of an effective model toward engineering applications in real set-tings of confined fluids such as chemical systems encountered in unconventional reservoirs (e.g., shale oil and shale gas).