Before the misinformation wheel starts spinning, it should be noted that ethylene is a naturally produced gas by all ripening fruits. This is why it "works" to put unripe bananas in a paper bag for a day or two to ripen them up. The bananas themselves are producing ethylene, which is a plant hormone that promotes ripening.

Many fruits and vegetables are harvested unripe and gassed with ethylene in the supply chain to "ripen" them as they are distributed, however, as correctly noted above this not as effective as ripening on the plant naturally, because while the fruit ripens on the plant it continues to accumulate sugar and other nutrients from the green parts of the plant. This is one of many reasons that garden grown tomatoes are always better tasting than store bought...most store bought tomatoes are gassed.