I live in a retirement community which is run by the Lutheran Church. Their liturgy is very similar to ours and they receive communion believing that they receive the body and blood of Christ. It obviously does not have the validity that our Mass has but how do you communicate this to them in a loving way?
These links should be helpful for understanding why Catholics should not take communion in a non-Catholic church. If you have any further questions or concerns that are not answered by these links, please contact Catholic Answers directly.