A flue gas passage arrangement for a steam generator which permits adjustment of the heat transfer effectiveness of a final bank of heat exchanger surface to control a temperature of the flue gas flowing through and exiting from the flue gas passage and conveyed to a downstream NOx reduction device having a minimum operating temperature. The NOx reduction device is advantageously an SCR. Economizer heating surface is located within the flue gas passage, and a baffle plate extends through the flue gas passage and creates two flue gas paths there through. The economizer heating surface may be a single, common bank having a section located in both gas paths, or in only one gas path. Alternatively, two separate banks of economizer heating surface may be employed. A variable position damper is provided in one gas path, either below or above one section. The variable position damper can be selectively opened or closed to permit or restrict the flow of flue gases through the second section to control the effectiveness of the economizer heating surface to maintain the temperature of the flue gas conveyed to the downstream NOx reduction device at or above the minimum operating temperature.