NOVELTY - An olefin cracking process involves passing condensed liquid removed from a refrigeration zone to a demethanation zone, and starting operation of the demethanation zone; after compression, refrigeration and demethanation zones have started up, starting up furnace and quench zones while introducing a natural hydrocarbonaceous feed into the furnace zone; and when a quenched gaseous product from the quench zone approaches normal operating conditions while employing the natural feed, terminating the introduction of artificial feed.    USE - The inventive process is used for cracking an olefin, e.g. ethylene, propylene, butenes, butadiene, and aromatics (e.g., benzene, toluene, and xylenes).    ADVANTAGE - The inventive process does not require flaring to the atmosphere for obvious conservation and environmental reasons.    DETAILED DESCRIPTION - An olefin cracking process comprises:    (A) employing a natural hydrocarbonaceous feed (40) in a furnace zone to cause thermal cracking of the natural feed to form a gaseous cracked feed product that contains molecular hydrogen, methane, and other hydrocarbonaceous compounds;    (B) passing the gaseous cracked feed product to a quench zone (22) to cool the gaseous cracked feed product and to separate a liquid fraction from a quenched gaseous product (24);    (C) passing the quenched gaseous product to a compression zone (25) to be compressed;    (D) cooling the compressed quenched gaseous product in a refrigeration zone (27) to condense to liquid state components of the natural feed that are desired to be recovered as a product of the cracking process, thus producing a mixture of the liquefied components and non-condensed gases including molecular hydrogen and methane;    (E) passing the mixture to a demethanation zone (29) for removal of the non-condensed gases;    (F) subsequently processing the liquefied components in a fractionation zone (33) to separately remove the liquefied components as final products of the cracking process, where the cracking process is not in a normal operational mode as to a temperature, a pressure and the natural feed throughput and is in need of start up to achieve the normal operational mode;    (G) maintaining the furnace and quench zones in the non-operational mode;    (H) maintaining the fractionation zone in an operating mode as to the temperature and the pressure and on total reflux;    (I) introducing an artificial feed gas into the compression zone for subsequent flow into the refrigeration and demethanation zones, where the artificial feed has a chemical composition different from the natural feed and being of a composition that approximates the composition of the natural feed to an extent that minimizes condensation of gaseous components to a liquid state in the compression, refrigeration and demethanation zones when compared to the condensation that would normally occur with the natural feed in the normal operational mode of the cracking process;    (J) starting compression of the artificial feed in the compression zone while continuing the introduction of artificial feed until the compression zone is at a pressure of greater than or equal to50% of its normal operational pressure;    (K) after reaching greater than or equal to50% of the normal operational pressure in the compression zone, starting cooling in the refrigeration zone while continuing the introduction of artificial feed to the compression zone;    (L) recycling all gases from the refrigeration zone to the input of the compression zone with no venting of the compression zone;    (M) continuing the compression and refrigeration zone start up with the gas recycling to the compression zone until the zones reach their normal operating conditions;    (N) while the compression and refrigeration zones are progressing toward their normal operating conditions, removing condensed liquid formed in the refrigeration zone from that zone and passing the liquid to the demethanation zone, and starting operation of the demethanation zone;    (O) after the compression, refrigeration and demethanation zones have started up, starting up the furnace and quench zones while introducing the natural feed into the furnace zone; and    (P) when the quenched gaseous product from the quench zone approaches normal operating conditions while employing the natural feed, terminating the introduction of artificial feed.    DESCRIPTION OF DRAWING(S) - The figure shows a block flow diagram of an olefin production plant when being started up.    Quench zone (22)    Quenched gaseous product (24)    Compression zone (25)    Refrigeration zone (27)    Demethanation zone (29)    Fractionation zone (33)    Natural hydrocarbonaceous feed (40)