A pressure proof reaction vessel and the method of making and operating such a vessel wherein the vessel has a thin inner wall whose primary function is to conduct heat and a thick outer wall whose primary function is to contain the pressure with said inner wall being supported by the said outer wall using closely-spaced supports between the two walls. Heat transfer fluid flows in channels formed by the two walls. A novel method of fastening together the inner and outer walls of such a vessel using fastening means associated with a hole passing through the outer wall and that do not introduce surface disruption of the inside surface of the inner wall, is disclosed. This fastening means also allows for a very economical fabrication method whereby the inner wall and the supports are formed out of one piece of material. The vessel is particularly useful for the polymerization of vinyl chloride in large reaction vessels greater than 10,000 gallons in size.