How? - Non-blocking XML message over HTTP

I’m trying to code up Rails so that it can talk to the UK Government
Gateway using the submission protocol described here

Essentially it is an asynchronous request/response system that sends XML
messages over an HTTPS socket - fairly standard stuff.

Now assuming nobody else has already written this thing, I’m going to
have to know together some sort of library to handle to protocol. I
don’t think the standard SOAP or XMLRPC stuff will do the job.

Writing it in a synchronous form is of course fairly straightforward,
however from experience I know that the Government Gateway can take an
enormous amount of time to respond to a request (even with a simple
acknowledgement message), and I don’t really want the controller action
to block waiting for the Gateway to respond.

Is there a best practice way to code this sort of thing up?

TIA

Regards

Neil W.
Aldur Systems, UK