In this study, numerical computations of condensation heat fluxes along a vertical flat plate from steam and air mixtures in a rectangular flow channel with a condensation height of 6 m were carried out using the computational fluid dynamics (CFD) code FLUENT to evaluate condensation heat fluxes q_c on the containment wall under accident conditions. The distributions of the computed heat flux q_x,CFD values along the flow direction x agreed relatively well with those of the q_x,exp values measured with the COPAIN facility with a condensation height of 2 m at a steam mass fraction of Y_s,in = 0.133–0.233, and the standard deviation of the q_x,CFD values to the q_x,exp values was s = 13%. The q_x,CFD values were numerically obtained with a mixture velocity range of u_in = 0.27–3.2 m/s at Y_s,in = 0.453, and they were compared with the q_c,NC values computed by existing correlations for natural convection (NC) condensation. A combination of a q_c correlation and a correlation for the Sherwood number Sh_x,NC in the q_c correlation was evaluated, the coefficient for Sh_x,NC was optimized, and the standard deviation of the q_x,NC values to the q_x,exp or q_x,CFD values was s = 16%.