BUENOS AIRES, Feb 23 (Reuters) - Argentina on Wednesday announced an average 20% increase in gas prices for residential customers and 15% for industry, at a time when annual inflation is around 50%. Metrogas MET.BA is the largest gas distributor in the country, while Edenor EDN.BA and Edesur are the main private power distributors. Public service subsidies are one of the largest expenses of Argentina's government and a key topic in its major debt renegotiation with the International Monetary Fund (IMF).