A method for producing gaseous ethylene oxide product from partial oxidation of hydrocarbon using a heterogeneous catalyst in a fixed bed enclosed within a reaction vessel shell, comprising: introducing a sufficient amount of gaseous ethylene, oxygen, ballast gas, and catalyst promoter into a reaction vessel having an inflow and an outflow, said vessel having a shell with a length and a width to define a reactor vessel volume; said reactor vessel volume defining a catalyst bed shape; wherein the catalyst bed is such that the cross sectional areas A1 and A2 at any two points of the catalyst bed separated by a length L1 between the in flow and the out flow are such that (A2-A1)/L1 is from 0.8 to 1.3m, said catalyst bed defining a process side and having a selectivity greater than 80%, said catalyst bed having a length less than said shell length and having a volume less than said reactor shell volume; circulating a heat transfer fluid through at least one coolant enclosure heat exchanger in said shell interior to define a coolant side of the process; said coolant enclosure equipped with an outside surface and an inside surface; said coolant enclosure outside surface in contact with said catalyst bed and having an inlet and an outlet for the flow of coolant fluid therethrough; said coolant enclosure further defining a cooling surface area with a coolant flow cross sectional area ratio to cooling surface area less than about 1; where the coolant enclosure is a tube; said coolant side having a higher pressure than said process side, and flowing said gaseous ethylene, oxygen, ballast gas and catalyst promoter over said catalyst bed and a gaseous ethylene oxide product through a fixed catalyst bed outlet zone in said reactor vessel; said zone configured with average residence time of less than or equal to 4 seconds of the gaseous ethylene oxide product flow from the outlet of the catalyst bed to a heat exchanger to quench said gaseous ethylene oxide product and flowing said gaseous ethylene oxide product through said reactor vessel outlet. introducing a sufficient amount of gaseous ethylene, oxygen, ballast gas, and catalyst promoter into a reaction vessel having an inflow and an outflow, said vessel having a shell with a length and a width to define a reactor vessel volume; said reactor vessel volume defining a catalyst bed shape; wherein the catalyst bed is such that the cross sectional areas A1 and A2 at any two points of the catalyst bed separated by a length L1 between the in flow and the out flow are such that (A2-A1)/L1 is from 0.8 to 1.3m, said catalyst bed defining a process side and having a selectivity greater than 80%, said catalyst bed having a length less than said shell length and having a volume less than said reactor shell volume; circulating a heat transfer fluid through at least one coolant enclosure heat exchanger in said shell interior to define a coolant side of the process; said coolant enclosure equipped with an outside surface and an inside surface; said coolant enclosure outside surface in contact with said catalyst bed and having an inlet and an outlet for the flow of coolant fluid therethrough; said coolant enclosure further defining a cooling surface area with a coolant flow cross sectional area ratio to cooling surface area less than about 1; where the coolant enclosure is a tube; said coolant side having a higher pressure than said process side, and flowing said gaseous ethylene, oxygen, ballast gas and catalyst promoter over said catalyst bed and a gaseous ethylene oxide product through a fixed catalyst bed outlet zone in said reactor vessel; said zone configured with average residence time of less than or equal to 4 seconds of the gaseous ethylene oxide product flow from the outlet of the catalyst bed to a heat exchanger to quench said gaseous ethylene oxide product and flowing said gaseous ethylene oxide product through said reactor vessel outlet. <EOS>