Abstract: First and second indirect contact condensing heat exchangers are used to separate gases from a mixture of condensible and non-condensible gases, such as the vapor from the last stage of a black liquor evaporation plant of a pulp mill. The mixture of gases passes over the first heat exchanger to cool and condense, producing a first condensate and a first remaining gas stream. The condensate is removed from the first remaining gas stream, the gas stream then being passed over the second heat exchanger. A medium for absorbing at least one non-condensible gas in the first gas stream--such as an NaOH liquid solutino--is passed over the surface of the second heat exchanger to absorb gas, and produce a second remaining gas stream (containing primarily air). The second gas stream is separated from the solution with absorbed gas. The solution with absorbed gas may be used as white liquor in kraft pulping in a pulp mill, without concentrating the solution.