Claus sulfur recovery plants that include one or more single-stage or multi-stage compact tubular Claus catalytic reactor-heat exchanger units are disclosed. In some instances, these new or improved Claus plants additionally include one or more compact heat exchanger containing cooling tubes that are filled with a heat transfer enhancement medium. The new compact tubular Claus catalytic reactor-heat exchanger units and HTEM-containing heat exchangers are also disclosed. A process for recovering sulfur from a hydrogen sulfide-containing gas stream, employing the new tubular Claus catalytic reactor-heat exchanger unit, and in some instances a HTEM-containing heat exchanger, are also disclosed.