An exhaust device for a combined cycle power plant, includes a diffuser and a plenum. The diffuser includes a first wall, a second wall, a diffuser inlet, a diffuser outlet, and a diffuser flow path. The first and second walls extend circumferentially about a centerline axis of the exhaust device. The second wall is spaced from the first wall. The diffuser flow path is defined between the first and second walls, and extends from the diffuser inlet to outlet. The plenum includes an inlet wall portion and a non-circular plenum outlet, where the inlet wall portion is coupled to the diffuser outlet. The non-circular plenum outlet is spaced from the diffuser outlet along an axial direction of the exhaust device.