The U.S. government allows dual citizenship. United States law recognizes U.S. Dual Citizenship, but the U.S. government does not encourage it is as a matter of policy due to the problems that may arise from it. It is important to understand that a foreign citizen does NOT lose his or her citizenship when becoming a U.S. citizen. An individual that becomes a U.S. citizen through naturalization may keep his or her original citizenship.
However, as some countries do not recognize dual citizenship, it is important to consider it carefully before applying for.
Therefore you really need to contact the PNG policy and legislation to ascertain if you are able to continue to hold your PNG passport once becoming a US citizenship. There was some debate in parliament some years ago and bill was to be passed to allow PNG citizenship with other countries, but not sure if this bill was passed.