In order to successfully study the condensation and separation of a steam–CO2 mixture, a boundary layer model was applied to the mixture condensation of steam and CO2 on horizontal and vertical plates. The modified condensation boundary layer model of steam and CO2, given the CO2 solubility in the condensate, was established, numerically solved, and verified with existing experimental data. Different condensation data of steam–air and steam–CO2 mixtures were compared, and the effect of CO2 solubility on the mixed gas condensation was analyzed under multiple pressure conditions (1 atm–10 MPa). The simulation data show that the presence of CO2 will deteriorate the condensation heat transfer, just like air. Given that CO2 is slightly soluble, some CO2 can pass through the gas–liquid interface to enter the condensate film and reduce the accumulated CO2 on the gas–liquid interface, which improves the condensation. However, the solubility of CO2 is only significant under high-pressure conditions, inducing its effects on condensation. A comparison of the condensation coefficients of the steam–CO2 mixture shows the lower impact of CO2 condensation on the horizontal plate compared to that on the vertical plate. For most conditions, the steam–CO2 mixture gas condensation heat transfer coefficient on the vertical plate surface is still larger than that on the horizontal plate surface, and the improvement in the condensation heat transfer coefficient caused by low CO2 solubility (2 or 10%) at 10 MPa on the vertical plate is also larger than that of the horizontal plate. [ABSTRACT FROM AUTHOR]