The phase behavior of fluids in nanopores deviates significantly from that in bulk space. However, the effect of pore confinement on the capillary condensation in nanopores has not been fully understood. In this work, the classic Kelvin equation is modified by incorporating the real gas effect, along with the pore size effect on the surface tension, the multilayer adsorption, and the molecule-wall interaction potential to improve its accuracy in calculating the capillary condensation pressure. The modified Kelvin equation is further extended for multicomponent fluids in nanopores. More specifically, an extended Peng-Robinson equation of state is applied to describe the real gas effect. The pore size effect on surface tension is reflected by accounting for the meniscus variation with pore size. The multilayer adsorption of both single- and multicomponent fluids are computed by the Brunauer-Emmett-Teller model, and the Frenkel-Halsey-Hill equation is used to calculate the molecule-wall interaction potential. Consequently, the modified Kelvin equation is validated with 42 collected experimental data, resulting in an overall relative deviation of 7.65 and 6.52% for single- and multicomponent fluids, respectively. It is also found that the molecule-wall interaction potential has the most significant contribution. Compared with the bulk condition, the capillary condensation pressure of CO2 at 265 K and the mixture CO2 + n-C5H12 + n-C6H14 at 390 K within 2 nm are predicted to be suppressed by 33.96 and 43.16%, respectively.