Reactor or heat exchanger with improved heat transfer performance

A shell-and-tube reactor or heat exchanger comprises first tubes, through the inside of which a first object for heat transfer with a heat transfer medium is passed, some of the first tubes being provided in a zone in which a flow of the heat transfer medium (parallel flow) parallel to the axis of the tubes is present; and a second tube, through the inside of which the first object is not passed, the second tube being provided in said zone such that it is parallel to the axis of the first tubes. A method for producing an oxide comprises using said shell-and-tube reactor or heat exchanger, and causing a catalytic vapor-phase oxidation reaction in first tubes, through the inside of which the first object for heat transfer with the heat transfer medium is passed.