Supplemental Medical Coverage

I have a pension of two companies, one is Tenneco. They found that I have Part D coverage by another company and are now canceling my supplemental health insurance from them.

Many companies that were providing cover for the group canceled when retired people have signed up for Medicare Part D, even if only for drugs. They have the right to do so and may require you to choose between the two. Sorry.

Deep is true, the insurer should have contacted you. Look if their letters, you must have a letter "creditible coverage" of some sorts that tells you how Part D affects your coverage.