I don't see how Queue can send a reply back. Do you mean there is some listener on the mainframe that listens to the 'QUEUEUNAME' and sends a reply back to the replyQueue?
It seems to me that the reply is not coming.
I'd suggest to simplify configuration to first simply validate that reply is coming by configuring a logger to the 'myReplyChannel'
Also, could it be that the mainframe is too slow with response? Try setting 'receive-timeout' attribute to some significant value. The default is 1 sec. so set it to 10 (e.g., receive-timeout="10000")
<si-jms:outbound-gateway id="mygateway" request-channel="splitchannel" request-destination="outboundQueue"
<si:logging-channel-adapter channel="myReplyChannel" level="WARN"/>