A vertical bundle air-cooled heat exchanger, In one embodiment, the invention can be a vertical bundle air-cooled condenser comprising: at least one tube bundle assembly comprising; a tube bundle comprising a plurality of finned tubes arranged in a substantially vertical and side-by-side orientation, each of the plurality of finned tubes comprising a cavity; a top header pipe comprising an inlet header cavity operably coupled to a source of steam; a bottom header pipe comprising an outlet header cavity for collecting condensate; top ends of the plurality of finned tubes coupled to the top header pipe and the bottom ends of the plurality of finned tubes coupled to the bottom header pipe; and, a shell having an open, top end and open bottom end, the at least one tube bundle assembly positioned within the shell.