A method and main heat exchange system for use in a cryogenic air separation plant in which down-flow and thermosiphon heat exchangers are employed to partially vaporize an oxygen-rich liquid produced in a lower pressure column and to condense the nitrogen-rich vapor in a higher pressure column. A greater proportion of the oxygen-rich liquid can be partially vaporized in the down-flow heat exchangers than in the thermosiphon heat exchangers and the nitrogen-rich vapor condensed in the thermosiphon heat exchangers can have a higher oxygen content than the nitrogen-rich vapor condensed in the down-flow heat exchangers. This allows the higher pressure column to operate at a lower pressure than would otherwise be possible. A central conduit can extend from the higher pressure column into the lower pressure column to introduce the nitrogen-rich vapor into at least the down-flow heat exchangers for purposes of reducing pressure drop and column height.