Molecular dynamics simulations are carried out to study the two-phase behavior of the n-decane + water system in the presence of methane, carbon dioxide, and their mixture under reservoir conditions. The simulation studies were complemented by theoretical modeling using the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) and density gradient theory. Our results show that the presence of methane and carbon dioxide decreases the interfacial tension (IFT) of the decane + water system. In general, the IFT increases with increasing pressure and decreasing temperature for the methane + decane + water and carbon dioxide + decane + water systems, similar to what has been found for the corresponding decane + water system. The most important finding of this study is that the presence of carbon dioxide decreases the IFT of the methane + decane + water system. The atomic density profiles provide evidence of the local accumulation of methane and carbon dioxide at the interface, in most of the studied systems. The results of this study show the preferential dissolution in the water-rich phase and enrichment at the interface for carbon dioxide in the methane + carbon dioxide + decane + water system. This indicates the preferential interaction of water with carbon dioxide relative to methane and decane. Notably, there is an enrichment of the interface by decane at high mole fractions of methane in the methane/decane-rich or methane/carbon dioxide/decane-rich phase. Overall, the solubility of methane and carbon dioxide in the water-rich phase increases with increasing pressure and temperature. Additionally, we find that the overall performance of the PC-SAFT EoS and the cubic-plus-association EoS is similar with respect to the calculation of bulk and interfacial properties of these systems.