A theoretical study of the problem of irreversible condensation in the general vapor was performed. Assuming a mechanism of the step-wise addition of molecules to clusters in the vapor by collisions under nonequilibrium steady-state conditions, a general rate expression for homogeneous condensation was derived. This general expression was then elucidated for two vapor types, of which sodium vapor and water vapor are examples. The results for a theoretical calculation, when compared with qualified experimental data for water vapor over a wide range of temperatures, indicated good agreement between theory and experiment.