A multi-tubular chemical reactor includes an igniter for the initiation of gas phase exothermic reaction within the gas phase reaction zones of the tubular reactor units. In accordance with the present disclosure, there is provided a multi-tubular chemical reactor comprising a plurality of spaced-apart reactor units, each reactor unit comprising an elongate tube having a wall with internal and external surfaces, an inlet at one end and an outlet at the opposing end, the wall enclosing a gaseous flow passageway at least a portion of which defines a gas phase reaction zone, the multi-tubular chemical reactor can include at least one igniter for initiation of at least one gas phase exothermic reaction within a gas phase reaction zone of a reactor unit.