A method of producing a pressurized product stream comprising: rectifying a feed stream containing oxygen and nitrogen by a cryogenic rectification process utilizing a main heat exchanger of plate-fin construction and a distillation column system operatively associated with the main heat exchanger; pumping a product stream withdrawn from the distillation column system and composed of oxygen-rich liquid or nitrogen-rich liquid to produce a pumped product stream; characterized in that the method further comprises: the main heat exchanger including a first set of layers and a second set of layers, each of the first set of layers and the second set of layers having first sections and second sections; warming subsidiary streams (104a, 104b) of at least part of the pumped product stream within the first sections of the first set of layers and the second set of layers of the main heat exchanger until the temperature of the subsidiary streams exceed the critical temperature or dew point temperature of the pumped product stream, while indirectly cooling one or more streams (24, 104a, 104b) associated with the cryogenic distillation process; combining the warmed subsidiary streams (104a, 104b) in the main heat exchanger to form a combined stream; and further warming the combined stream within the second section of the first set of layers of the main heat exchanger to produce the pressurized product stream while indirectly warming or cooling another stream (30b, 114b) associated with the cryogenic distillation process within the second sections of the second set of layers. rectifying a feed stream containing oxygen and nitrogen by a cryogenic rectification process utilizing a main heat exchanger of plate-fin construction and a distillation column system operatively associated with the main heat exchanger; pumping a product stream withdrawn from the distillation column system and composed of oxygen-rich liquid or nitrogen-rich liquid to produce a pumped product stream; characterized in that the method further comprises: the main heat exchanger including a first set of layers and a second set of layers, each of the first set of layers and the second set of layers having first sections and second sections; warming subsidiary streams (104a, 104b) of at least part of the pumped product stream within the first sections of the first set of layers and the second set of layers of the main heat exchanger until the temperature of the subsidiary streams exceed the critical temperature or dew point temperature of the pumped product stream, while indirectly cooling one or more streams (24, 104a, 104b) associated with the cryogenic distillation process; combining the warmed subsidiary streams (104a, 104b) in the main heat exchanger to form a combined stream; and further warming the combined stream within the second section of the first set of layers of the main heat exchanger to produce the pressurized product stream while indirectly warming or cooling another stream (30b, 114b) associated with the cryogenic distillation process within the second sections of the second set of layers. the main heat exchanger including a first set of layers and a second set of layers, each of the first set of layers and the second set of layers having first sections and second sections; warming subsidiary streams (104a, 104b) of at least part of the pumped product stream within the first sections of the first set of layers and the second set of layers of the main heat exchanger until the temperature of the subsidiary streams exceed the critical temperature or dew point temperature of the pumped product stream, while indirectly cooling one or more streams (24, 104a, 104b) associated with the cryogenic distillation process; combining the warmed subsidiary streams (104a, 104b) in the main heat exchanger to form a combined stream; and further warming the combined stream within the second section of the first set of layers of the main heat exchanger to produce the pressurized product stream while indirectly warming or cooling another stream (30b, 114b) associated with the cryogenic distillation process within the second sections of the second set of layers.