A heat exchanger for cooling a fluid with a mixed refrigerant, comprising: a warm end and a cold end; a feed fluid cooling passage having an inlet at the warm end and adapted to receive a feed fluid, and having a product outlet at the cold end through which product exits the feed fluid cooling passage; a primary refrigeration passage (104, 204) having an inlet at the cold end and adapted to receive a cold temperature refrigerant stream, a refrigerant return stream outlet at the warm end through which a vapor phase or mixed phase refrigerant return stream exits the primary refrigeration passage, and an inlet adapted to receive a middle temperature refrigerant stream and located between the cold temperature refrigerant stream inlet and the refrigerant return stream outlet; a high pressure vapor passage adapted to receive a high pressure vapor stream at the warm end and to cool the high pressure vapor stream to form a mixed phase cold separator feed stream, and including an outlet and a cold vapor separator, wherein the outlet is in communication with the cold vapor separator, the cold vapor separator adapted to separate the cold separator feed stream into a cold separator vapor stream and a cold separator liquid stream; a cold separator vapor passage having an inlet in communication with the cold vapor separator and adapted to condense and flash the cold separator vapor stream to form the cold temperature refrigerant stream, and having an outlet in communication with the primary refrigeration passage inlet at the cold end; a cold separator liquid passage having an inlet in communication with the cold vapor separator and adapted to subcool the cold separator liquid stream, and having an outlet and a middle temperature refrigerant passage, wherein the outlet is in communication with the middle temperature refrigerant passage; a high pressure liquid passage adapted to receive a mid-boiling refrigerant liquid stream at the warm end and to cool the mid-boiling refrigerant liquid stream to form a subcooled refrigerant liquid stream and having an outlet in communication with the middle temperature refrigerant passage; and the middle temperature refrigerant passage adapted to receive and combine the subcooled cold separator liquid stream with the subcooled refrigerant liquid stream to form a middle temperature refrigerant stream, and having an outlet in communication with the primary refrigeration passage inlet adapted to receive the middle temperature refrigerant stream. a warm end and a cold end; a feed fluid cooling passage having an inlet at the warm end and adapted to receive a feed fluid, and having a product outlet at the cold end through which product exits the feed fluid cooling passage; a primary refrigeration passage (104, 204) having an inlet at the cold end and adapted to receive a cold temperature refrigerant stream, a refrigerant return stream outlet at the warm end through which a vapor phase or mixed phase refrigerant return stream exits the primary refrigeration passage, and an inlet adapted to receive a middle temperature refrigerant stream and located between the cold temperature refrigerant stream inlet and the refrigerant return stream outlet; a high pressure vapor passage adapted to receive a high pressure vapor stream at the warm end and to cool the high pressure vapor stream to form a mixed phase cold separator feed stream, and including an outlet and a cold vapor separator, wherein the outlet is in communication with the cold vapor separator, the cold vapor separator adapted to separate the cold separator feed stream into a cold separator vapor stream and a cold separator liquid stream; a cold separator vapor passage having an inlet in communication with the cold vapor separator and adapted to condense and flash the cold separator vapor stream to form the cold temperature refrigerant stream, and having an outlet in communication with the primary refrigeration passage inlet at the cold end; a cold separator liquid passage having an inlet in communication with the cold vapor separator and adapted to subcool the cold separator liquid stream, and having an outlet and a middle temperature refrigerant passage, wherein the outlet is in communication with the middle temperature refrigerant passage; a high pressure liquid passage adapted to receive a mid-boiling refrigerant liquid stream at the warm end and to cool the mid-boiling refrigerant liquid stream to form a subcooled refrigerant liquid stream and having an outlet in communication with the middle temperature refrigerant passage; and the middle temperature refrigerant passage adapted to receive and combine the subcooled cold separator liquid stream with the subcooled refrigerant liquid stream to form a middle temperature refrigerant stream, and having an outlet in communication with the primary refrigeration passage inlet adapted to receive the middle temperature refrigerant stream.