Q. Are Catholics allowed to receive Communion in non-Catholic churches?
A. No, they are not. Receiving Communion in a non-Catholic church would proclaim that a person is in “communion” with that denomination and its teachings, and therefore not in “communion” with the Catholic Church.
Moreover, Communion for Protestant Christians is not what it is for Catholics. We believe that the most Holy Eucharist is truly the Body, Blood, Soul, and Divinity of Jesus Christ, and this is true because bishops who enjoy an unbroken continuity with the original apostles ordained by Christ have validly ordained our priests. Protestants, on the other hand, do not have this “Real Presence,” as we say, in their Communion; nor do most of them even believe in such a Real Presence in Communion.
The only time that a Catholic may licitly receive the Eucharist in a non-Catholic Church is spelled out clearly in Canon 844.2: “Whenever necessity requires or a genuine spiritual advantage commends it, and provided the danger of error or indifferentism is avoided, Christ’s faithful for whom it is physically or morally impossible to approach a Catholic minister, may lawfully receive the sacraments of penance, the Eucharist and anointing of the sick from non-Catholic ministers in whose Churches these sacraments are valid” (emphasis added).
It is commonly understood that the churches in which these sacraments are valid are the Eastern Orthodox churches (Greek Orthodox, Byzantine Orthodox, etc.), the Assyrian Church of the East, and the Polish National Catholic Church. But the Catholic Church does not consider Communion in the Protestant denominations to be valid (that is, the true Body and Blood of Christ), however meaningful the ceremony may be to those who take part in it.