Hydrocarbon streams are heated uniformly and directly using one or more flameless oxidation burners. Flameless oxidation burners may control the temperature within a range of about 50° C. to reduce the tendency to coking due to localized “hot spots”. The hydrocarbon streams, which may be monomer streams, may pass cross-current (perpendicular) to burners having a longitudinal design or configuration, or may pass parallel to such burners.