A heat exchanger includes a casing configured to direct a working fluid therethrough, and at least one heat exchanger (HE) section in the casing. Each HE section includes a pair of spaced supports. The spaced supports include: an upstream support and a downstream support with at least one of them including a coolant carrying body configured to direct a coolant therethrough. A first cross-support couples to and extends between respective upstream and downstream supports; and at least one second cross-support couples to and extends between the respective upstream and downstream supports. Cross-supports are vertically distanced from adjacent cross-supports. A plurality of tube positioners coupled to each cross-support position a plurality of heat exchange tubes extending across a working fluid path through the casing. The tube positioners and the cooling of the cross-supports allows ferritic material to be used for once-through, duct-fired HRSGs.