For what?

Heinemann, which is a beauty BTW, coudn't be farther away from a serif. Why would you want to pair it with one?

To help people read long text better?

hhp

Do you want the same ‘early reading’ letter shapes (i.e. one-storey ‘a’ and ‘g’)?

No, hrant, only sans serif fonts designed to be used in telephone books are allowed for running text now. Hadn't you heard? :D

Which explains how telephone books are treated these days. ;-)

hhp

Awwww, booo! :D

If you want the same early reading/infant/schoolbook forms, Mark Simonson's Bookmania has alternates of ‘a’ and ‘g’ that could match Heinemann’s.

But we can't recommend much if we don't have some idea of the eventual use.

Stone Informal seems to fit

Maybe a blockish serif like Serifa would fit? That would provide a nice contrast with the roundish Heinemann. (BTW I mean Frutigers font, not any other ones named the same.)