I don't know if an automotive emissions analyzer would cover all the gases of interest, but it might be a start. The instrument makers Bacharach and Beckmann might offer some good choices, maybe even off-the-shelf what you need. Web searching "flue gas analyzers" could turn up some leads also. I haven't run a boiler in some years, but some of the boiler folks here can probably suggest.

It's best to install the sensor in the outlet flue before the tip of the exhaust stack.