The　supersonic　condensation　process　of　a　ternary　mixture,　including　methane,　water　vapor　and　nonane,　in　a　nozzle　was　studied　in　order　to　understand　the　condensation　mechanism,　nucleation　and　droplet　growth　rules　of　water　vapor　and　heavy　hydrocarbons　from　natural　gas　mixture.　Firstly,　based　on　the　existing　mathematic　model　of　the　two-component　mixture　supersonic　condensation　and　combined　with　the　latest　research　results　at　home　and　abroad,　a　physical　model　for　the　condensation　process　of　the　ternary　mixture　in　supersonic　flows　was　built.　Secondly,　a　mathematic　model　of　the　ternary　mixture　supersonic　condensation　was　built　and　applied　in　analog　computation　according　to　simulation　results　of　the　two-component　mixture　supersonic　condensation.　The　result　showed　that　in　the　ternary　mixture,　water　vapor　begins　to　condensate　spontaneously　prior　to　nonane　vapor　(x=4.0　mm),　forming　droplets　to　serve　as　allochthonous　nuclei　for　nonane　vapor　condensation　and　lowering　the　free　energy　barrier　of　nonane　vapor.　Consequently,　nonane　vapor　begins　to　condensate　at　a　lower　subcooling　(38.3　K)　and　supersaturation　(37.4).　In　other　words,　the　occurrence　of　water　vapor　in　a　sense　accelerates　the　condensation　process　of　nonane　vapor.