A multi-tubular chemical reactor (400) includes an igniter (435) for the initiation of gas phase exothermic reaction within the gas phase reaction zones (409) of the tubular reactor units (408). A method of carrying out a gas phase exothermic reaction within the multi-tubular chemical reactor comprising: introducing gaseous reactants into a tubular reactor unit (408); initiating with radiant heat an exothermic reaction of the gaseous reactants within the reactor unit; and transferring heat produced by the exothermic reaction occurring within the gas phase reaction zone of the reactor unit to the gas phase reaction zone of one or more adjacent reactor units (408), thereby initiating an exothermic reaction within at least one adjacent reactor unit (408) until in such manner an exothermic reaction has been initiated in each of the plurality of spaced-apart reactor units (408).