A syngas production system includes a gasification reactor and a syngas pressure vessel downstream of the gasification reactor. The syngas pressure vessel includes a pressure vessel having a body with a first portion and a second portion. The syngas pressure vessel also includes an evaporator disposed in the pressure vessel; a coil disposed in the pressure vessel; and a tongue-and-groove flange assembly. The tongue-and-groove flange assembly includes: a first flange with a raised ring extending from a face of the first flange, the first flange attached to the first portion of the body; a second flange with a groove defined in a face of the second flange. The second flange is attached to the second portion of the body. The raised ring extends from the face of the first flange and is positioned in the groove defined in the face of the second flange.