A heat insulation and corrosion protection of the inner vessel wall of a heat exchanger in a synthesis gas cooler. The heat insulation and corrosion protection are arranged above a thermal seal fastened between the vessel wall and the cooling tubes. The insulation and protection includes segments of fiber-filled plates, which are fastened to the vessel wall by means of a substructure and an adhesive or foam material. Support rings with a heat insulation protection means absorb the vertical forces. A metallic corrosion protection layer is applied to the inner vessel wall under the thermal seal. Free breathing of the vessel wall is possible and pressure equalization between the gas space and the annular space is guaranteed due to the heat insulation and corrosion protection according to the present invention.