NOVELTY - A reactant effluent stream processing system comprises ducts (30); a condenser (16) for condensing water out of the humidified gaseous reactant effluent stream; a separator for separating water from the gas in the condenser effluent stream; and a vent (24) for venting the gas from the separator, comprising a layer of antifreeze through which the gas in the separator can pass to ambient surroundings.    USE - The reactant effluent stream processing system is useful in proton exchange membrane fuel cell power plant having fuel cells that produce humidified gaseous reactant effluent stream during power plant operation. It is particularly useful in mobile applications, i.e. proton exchange membrane fuel cell-powered automobiles and buses.    ADVANTAGE - The reactant effluent stream processing system has a passive air vent through which air separated from the cathode effluent stream can be expelled from the power plant. The air vent operates satisfactorily during ambient freezing conditions, making it suitable for mobile applications.    DETAILED DESCRIPTION - A reactant effluent stream processing system comprises ducts (30) where humidified gaseous reactant effluent stream flows away from the fuel cells (4); a condenser (16) for condensing water out of the humidified gaseous reactant effluent stream so as to produce a condenser effluent stream which contains a gas and condensed water; a separator for separating water from the gas in the condenser effluent stream; and a vent (24) for venting the gas from the separator, comprising a layer of antifreeze through which the gas in the separator can pass to ambient surroundings. At least part of the water in the reactant effluent stream is the result of evaporative cooling of the fuel cells with water which vaporizes from a water coolant flow field into a reactant stream on a cathode side of the fuel cells. The antifreeze and condensed water are immiscible so that water separates from the antifreeze layer in the vent. The vent comprises a hydrophobic porous material layer supporting the antifreeze layer. The material layer allows passage of gases but prevents passage of liquids. The gas from the separator passes through the material layer into the antifreeze layer. An INDEPENDENT CLAIM is included for a method for removing air from a humidified reactant effluent stream produced in a proton exchange membrane fuel cell power plant cell stack.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view of a proton exchange membrane fuel cell assembly used in power plant.    Fuel cell (4)    Anode side (8)    Oxidant flow field (10)    Coolant flow field (12)    Condenser (16)    Vent line (22)    Vent (24)    Ducts (30)    Pump (32)