Molecular dynamics simulations were conducted to investigate the bulk and interfacial properties of alkanes in the presence of CH4, CO2, and their mixture under reservoir conditions. Theoretical analysis based on the predictive Peng-Robinson equation of state in combination with density gradient theory was used for comparison with the simulation results. Linear, branched, and cyclic alkanes (C7-C19) were considered for this study. We observed preferential dissolution in the alkane-rich phase and accumulation in the interfacial region of CO2 from the CH4/CO2 mixture. The solubility of CH4 and CO2 generally decreases with the number of carbon atoms in the alkane molecule n and are relatively lower in the presence of cycloalkanes. Our results show that at a fixed temperature and pressure, the interfacial tension (IFT) values of the CO2 + alkane system increase with the addition of CH4. These IFTs increase with n and are relatively higher in the presence of cycloalkanes. The cyclization effects are more important than branching effects for the bulk and interfacial properties of the studied systems. Our results may help to improve the understanding of the effects of impurities like CH4 on CO2-enhanced oil recovery under reservoir conditions.