A heat exchanger includes a plurality of heat exchange tubes arranged into a first row of tubes and a second row of tubes. A fixed mount fixedly positions the first row of tubes, and a movable mount makes the second row of tubes movable between an aligned position in which tubes in the second row are aligned with tubes in the first row, creating a linear flow path for the fluid through the tubes, and a non-aligned position in which tubes in the second row are not aligned with tubes in the first row, creating a curvilinear flow path for the fluid through the plurality of tubes. Heat is exchanged between the first fluid and a second fluid passing through the plurality of heat exchange tubes. The heat exchanger may be employed as an HRSG in a combined cycle plant, among other applications.