Production optimization from shale gas resources is a challenging task, especially when the knowledge of the fluid-phase behavior is not sufficient. Due to nanoscale pores of the shale matrix and higher degree of interactions between the molecules and fluid-pore wall compared to a conventional reservoir, the phase behavior of the shale fluid under confinement is significantly different from that of the bulk fluids observed in a PVT cell. The so-called nanoconfinement effect causes a shift in the critical properties, which could be as high as 60%, and shrinks the phase diagram to a large extent; therefore, correction of thermodynamic properties is vital for accurate reserve estimation and reservoir engineering calculations. In this study, we investigate the effect of confinement on the phase behavior of multicomponent gas in shale media through detailed numerical simulations. The nanoconfinement is accounted for at different levels of pore radii and intrinsic permeabilities in the presence of several major contributing mechanisms to shale gas flow, including but not limited to viscous flow, Knudsen diffusion, gas slippage, pore size variation, and adsorption. The phase envelopes as well as temporal and spatial variations of the composition in the shale matrix block are obtained and analyzed. The theoretical framework and analysis presented herein shed light on the phase behavior of the confined fluid, and the model can be used in shale and tight gas reservoir simulations.