Gas turbine engine heat exchangers and methods of assembling the same

A heat exchanger assembly (130) comprises a heat exchanger body (202) including a first fluid circuit (100) and a second fluid circuit (102). The first circuit includes a first bypass valve (136) in flow communication with a first fluid circuit inlet channel (248). The first fluid circuit also includes a plurality of cooling channels (232) in flow communication with the first bypass valve. The first bypass valve is configured to deliver a first fluid to the plurality of cooling channels during a first mode of operation to facilitate reducing a temperature of the first fluid. The second fluid circuit includes a second bypass valve (137) configured to facilitate a flow of a second fluid through at least a portion of the heat exchanger body during the first mode of operation.