A water gas shift reactor (10) in which the shift reactor is maintained at near constant temperatures and/or is continuously humidified in manners that attain low exit temperatures at elevated humidity in order to achieve minimum carbon monoxide concentration in the exit gas through exit tube (60). Heat exchanger (10) comprises a housing (12) with a chamber (14). The chamber (14) holds upper and lower circular tubes (16) and (18), each with passages (20) and (22) respectively. The tubes (16) and (18) are connected by vertical tubes (24) and passages (20) and (22) are connected with vertical passage (26). Side arm tube (36) with passage (38) permits water vapor within passage (30) and (34) while pump (40) facilitates circulation.