Pemex said the man, a worker of the contracting firm Atzco, was given immediate medical attention but had no vital signs by the time he arrived at a hospital.
The company said in a statement that the plant at the Ciudad Madero refinery located in northern Tamaulipas state has been isolated to attend to the leak.
The Mexican oil giant said its plants were operating normally and that no-one was injured.
The Salamanca refinery is located in central Guanajuato state and is Pemex's fifth biggest plant.