You need to use PublishTransactionsFilterData
and then either GetSystemWideListXml
or GetSystemWideList
methods:
var filter = new PublishTransactionsFilterData();
filter.PublicationTarget = new LinkToPublicationTargetData{ IdRef = "tcm:x-xx-xxx"};
client.GetSystemWideListXml(filter);
client.GetSystemWideList(filter);